1. Field of the Invention
The present invention relates to a battery module in which a plurality of prismatic batteries are retained in a compressed state.
2. Description of Related Art
A battery module has a laminate type casing member and incorporates a plurality of prismatic lithium-ion batteries accommodated in the casing member, each having a plurality of electrode plates laminated with separators sandwiched between them. This type of battery module has a large capacity and shows good performance at high rate current. For this reason, the battery has been used for robots, electric vehicles, and backup power sources, for example.
In the laminate type prismatic lithium-ion batteries such as described above, expansion and shrinkage of the electrode plates occur during charge and discharge. When the distance between the electrode plates increases, electron conductivity lowers in the positive electrode active material layer and the negative electrode active material layer, and as a consequence, the internal resistance increases, causing battery capacity loss and cycle performance deterioration. In view of this, in the battery module in which the prismatic lithium-ion batteries are stacked, it is desirable that each of the batteries is compressed so that the distance between the electrode plates cannot increase.
In Japanese Published Unexamined Patent Application Nos. 2003-323874, 2005-259500, and 2006-196222, the laminate type prismatic lithium-ion batteries as described above are stacked in a thickness direction with plates or spacers interposed therebetween. Constraining plates are provided at both ends of the stacked batteries, and the constraining plates are clamped with a belt or connecting rods so that each of the batteries is compressed.
A problem with such methods of compressing the batteries is that it is difficult to apply a uniform pressure to each of the batteries. In particular, when the number of the stacked batteries is large, the problem arises that the pressure applied tends to vary between the batteries positioned at both ends and those positioned in the vicinity of the center. Thus, the internal resistance rises in some of the batteries, and consequently, battery capacity loss and cycle performance deterioration tend to occur easily.
During high rate discharge, each of the batteries generates heat, but in the conventional battery module structures, the heat cannot be dissipated sufficiently. In some cases, the battery temperature rises to the upper limit of the operating temperature range, causing the battery to fail to discharge.
In Japanese Published Unexamined Patent Application No. 2006-40696, a plurality of flat-shaped batteries is stacked in a thickness direction, and a set of heat sinks are provided at both ends along the stacking direction. By bringing a set of fastening plates provided at side faces close to each other, the heat sinks are shifted so that they are brought close to each other, whereby the batteries are compressed.
This type of battery module also has the problem that it is difficult to apply a uniformly pressure to each of the batteries because the batteries are pressed by the heat sinks disposed at both ends. Moreover, heat dissipation from each of the batteries is not sufficient, so the problem of the battery temperature elevation also remains unresolved.